This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
Currently, wireless communication networks or systems operating at high frequencies from 30-300 GHz are emerging as a promising technology to meet exploding bandwidth requirements by enabling multi-Gb/s speeds. For example, the 5th Generation (5G) network is likely to be a combination of evolved 3rd Generation (3G) technologies, the 4th Generation (4G) technologies and emerging or substantially new components such as Ultra-Density Network (UDN), which is also referred to as MMW Radio Access Technology (RAT). At such high frequencies, a large number of antennas can be available at a transmitter, a receiver, or both. In order to make up for the large propagation loss that typically occurs, beam-forming becomes a very important feature in MMW wireless systems.
Beam-forming is a signal processing technique used for directional signal transmission and/or reception. For Transmitter (TX) beamforming, the signals are concentrated in the desired direction via applying a selected precoding vector for the TX antenna array. For Receiver (RX) beamforming, the RX beam of the receiver antennas are concentrated in the incoming direction of the radio signals by applying a selected precoding vector for the RX antenna array. Beam-forming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity. The improvement compared with omnidirectional reception/transmission is known as a beam-forming gain. When multiple antennas are available at transmitters, receivers or both, it is therefore important to apply efficient beam patterns to the antennas to better exploit the spatial selectivity of the corresponding wireless channel.
FIG. 1 schematically shows one example MMW RAT network. As shown in FIG. 1, there is a network node or a control node called as Central Control Unit (CCU), which is at least responsible for parameter configurations and coordination among Access Nodes (ANs), e.g., AN1, AN2, AN3, and AN4.
Typically, received power in a receiver side of a link can be expressed as:
      P    rx    =            P      TX        ·          G      TX        ·                            G          RX                ⁡                  (                      r                          4              ⁢                                                          ⁢              πλ                                )                    2        ·          e                        -          α                ⁢                                  ⁢        r            where PTX is transmitted power from a transmitter side of the link, GTX and GRX are beamforming gains of transmitting and receiving antennas, respectively, λ is the wavelength, and a is the attenuation factor due to absorption in the medium. For an MMW-wave link at 60 GHz, oxygen absorption loss can be as high as 16 dB/km.
From the above formula, it is clear that the attenuation of radio wave is proportional to 1/λ2. With the same propagation distance, 60 GHz attenuates 29.5 dB more compared to 2 GHz, without considering the oxygen absorption.
In considering this, high gain beam-forming is mandatory in order to compensate the extra attenuation. Thanks to the small wavelength, more antenna elements can be integrated in the antenna panel with the same size. This makes it possible to reach a higher beam-forming gain. However, if there are several tens or several hundreds of antenna elements, one Radio Frequency (RF) chain (either TX RF chain or RX RF chain) for each antenna element is inapplicable due to unacceptable cost. In such a case, multiple antenna elements share one RF chain and a specific analog phase adjustment is applied for each antenna in order to adjust the beam direction and maximize the beam-forming gain. Due to the narrow TX beam, it is needed to steer transmission of beacon signals to enable AN discovery area, and to preform beam-forming training to maximize the beam-forming gain.
Meanwhile, high gain beam-forming could bring challenges, including, e.g., hidden node problem and deafness problem. Those problems will be described in detail hereafter.
FIG. 2 illustrates an example of the hidden node problem caused by directivity of high gain beam-forming. As shown in FIG. 2, link 1 is composed by Access Point 1 (AP1) and User Equipment 1 (UE1), and link 2 is composed by AP2 and UE2. When AP2 is transmitting to UE2, neither AP 1 or UE 1 can detect the channel utilized by AP2 and UE2 because both AP1 and UE1 are outside of the TX beam coverage from AP2 to UE2. However, when AP1 transmits data to UE1, its TX beam can reach UE2 and cause interference.
FIG. 3 illustrates an example of the deafness problem caused by directivity of high gain beam-forming. As shown in FIG. 3, UE 1 and AP1 compose link 1 and UE2 and AP2 compose link 2. The link 2 has ongoing data transmission from AP2 to UE2. But this is not detected by UE1 because UE1 does not monitor (or sense) this direction. However, when UE 1 starts data transmission, the data receiving by UE2 can be clearly impacted due to UE1 and UE2 are close to each other.
Currently, it is supposed that the total carrier bandwidth of the MMW-RAT can be up to 1 or 2 GHz. This bandwidth can be composed by a number of sub-band carriers of a certain bandwidth, e.g. 100 MHz. By way of example, FIG. 4 illustrates one MMW-RAT carrier with 4 sub-bands. The smallest resource grid in FIG. 4 corresponds to a sub-band in the frequency domain and to a subframe in the time domain, and may be referred to as a sounding and sensing resource element. Of course, the sounding and sensing resource element may be also in terms of code.
To allocate the available resources, a contention based resource allocation scheme and/or a scheduling based resource allocation scheme may be applied in MMW-RAT as the basic policy of collision avoidance. A contention based resource allocation scheme provides a mechanism to compete for the channel based on the self-determination on the channel availability. In a scheduling based resource allocation scheme, a scheduler, e.g., a CCU as shown in FIG. 1, gains the resource controllability first via either contention based method or coordination method first and allocates the resource to controlled links.
There could be certain combination of the contention based resource allocation scheme and the scheduling based resource allocation scheme. FIG. 5 illustrates an example of a complex interference situation in a MMW-RAT network. As shown in FIG. 5, due to directivity of high gain beam-forming, Link 1 and Link 2 may have unendurable UpLink (UL) to DownLink (DL) interference while Link 5 and Link 6 may have unendurable DL to DL interference and UL to DL interference. Such interference may be referred to inter-link interference hereinafter.
In addition to links within a MMW RAT network, the inter-link interference may further involve links between two MMW RAT networks operating at the same frequency spectrum. For sake of simplicity, two networks operating at the same frequency spectrum may be called as spectrum sharing networks, which are typically, e.g., two partially overlapping, adjacent or neighboring (i.e., with certain distance in-between) networks. In regard of this, the inter-link interference within one network (e.g., the interference as shown in FIG. 5) may be referred to as intra-network inter-link interference, while the inter-link interference between two spectrum sharing networks may be referred to as inter-network inter-link interference hereinafter. That is, the inter-network inter-link interference will predominantly occur between two spectrum sharing networks.
FIG. 6 shows a typical scenario of inter-network inter-link interference between two spectrum sharing MMW RAT networks. It is assumed that there are several MMW RAT networks in such a scenario, including Network A and Network B operating at the same frequency spectrum. There is a link (Link A) from AN 1 to UE 1 in Network A, and there is a link (Link B) from AN 2 to UE 2 in Network B. When AN 1 and AN 2 are transmitting respective sounding signals simultaneously (at the same time, both of UE 1 and UE 2 are sensing sounding signals, i.e., being in RX state), AN 1's TX beam coverage partly overlaps with UE 2's RX beam coverage, as illustrated. This is, Link B is interfered by Link A.
Due to directivity of high gain beam-forming, the collision determination is more complex than omni-transmission. The traditional measurement does not work well due to the aforementioned deafness and hidden node problems. Besides, though carrier sensing methods commercially used in Wireless Local Area Network (WLAN, 802.11) and Wireless Personal Area Network (WPAN, 802.15) are developed, they are mainly for local access system. It is a distributed carrier sensing scheme, i.e., the carrier sensing is done by each node independently. For MMW RAT, firstly it is expected that there can be better dimensioned deployment involving multiple nodes of APs and UEs, and better network controllability (e.g., self-optimization, self-organization, and mobility) than Wireless Fidelity (WiFi) is targeted. Secondly, MMW RAT is expected to provide much better Quality of Service (QoS) than WiFi. In this sense, a better measurement than simple distributed carrier sensing of WiFi is desired.
The interference measurements in 3G and 4G wireless systems are mainly designed to measure the inter-cell/inter-Transmission-Point interference, rather than inter-link interference (including the intra-network inter-link interference and the inter-network inter-link interference). Due to small sector size and the large overlapping coverage in case of MMW RAT, the similar measurement as 3G or 4G systems is not enough to identify links in collision and help the interference management.